I'm puzzled by the "a clear path to the target" rule.

To target something, you must have a clear path to it, so it can't be behind total cover.

This seems straightforward: you can't target a point/creature/object/thing behind total cover. But then the rule adds the following:

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

And now suddenly I'm lost. You can't do A, but if you do, then B happens.

Let's assume a blind wizard stuck is in a maze, facing a wall he's not aware of. He's trying to cast fog cloud behind it.

How should the DM react, according to the rules?

I mostly see 3 options (but feel free to add others):

1. 
   

   "No you can't. Choose another target / action."

   
   
   

   This is the strict application of the rule's first sentence.

   
   


2. 
   

   "Even though you placed your spell 60 feet away, you feel a strong humidity on your skin. It looks like the fog cloud appeared closer than what you intended."

   
   
   

   This would be the application of the rule's second sentence.

   
   


3. 
   

   "You say the words and do the gestures, but nothing happens."

   
   
   

   As found here, Xanathar's Guide to Everything offers optional rules for invalid targets — that you can use... if you feel this falls into invalid targeting.

   
   

What bothers me is that the first option seemed to have an broad consensus (~90 person before the associated answer update): not only does it render the second sentence obsolete, but it allows the player to gain information on it's environment without spending actions. Suddenly, the wizard gains some kind of radar that allows him to check if the path in front of him is free, and circulate quickly in a maze made of prismatic walls without harm. This can't be right... right?

#2 is correct in this case.

The relationship between those two sentences can be resolved as You can't do A. If you try to do A, B happens instead. You can't place the fog cloud behind the unseen wall, and if you try, it ends up in front of the wall instead.

But all of these outcomes are appropriate in some cases.

The rule about the effect ending up on the near side of the obstruction only applies when you're placing an area of effect, not when the spell targets a single creature or object.

"No, you can't" (#1) is correct when the player tries to choose a target that his character has no way to even identify. If our blind wizard tries to cast magic missile, well, no, he can't. Because it has to be cast at a creature you can see.

"You do the spell but nothing happens" (#3) is correct when the player chooses a target that is invalid for a reason the character doesn't know yet. Like, if you cast magic missile at a monster who's actually an illusion, or try to cast cure wounds on someone you don't know is a simulacrum.

#2 - Fog cloud would appear on the wizard's side of the wall

To target something, you must have a clear path to it, so it can't be behind total cover.

This gives a broad, general rule that covers anything that targets something. However, rules then go on to create a special case that tells you how to apply the rule in a specific circumstance:

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This special case applies only to an AOE cast while the caster cannot see. You still cannot target the other side of the obstruction so you are still following the top line. The rule is simply telling you what happens after you try to target it. In this case, the spell simply comes into being on the near side of the obstruction where it follows all the normal rules.

So, in the example you give, the blind wizard would try to target a point behind a wall with fog cloud. The rules do not allow targeting that point and instead say that the effect comes into being on the near side of the wall which is what happens.

There's no contradiction at all here between the first and second lines of the spell or how they interact.

With a seeing wizard or non-AOE spells the other two cases are possible

What about non-AOE spells and the rule from Xanathar's?

A spell that targets a creature or object also still follows the general rule that the covered point cannot be targeted, but obviously the second line does not apply. The only difference between this case and the blind-AOE case is that the rule does not tell us what happens in this case.

This is where the optional rule from XGE comes in. It gives the DM an optional way to deal with attempts to cast a spell on an invalid target since this case is not defined in the original rule. Which means that, depending on the specific DM and case, either #1 or #3 would still be valid rulings for non-AOE spells casted by a non-blind spellcaster.

As others have stated, #2 is the answer for your situation.

The answer is already in the wording:

To target something, you must have a clear path to it, so it can't be behind total cover.

and

If you place an area of effect at a point that you can't see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

In the Player's Handbook: Spellcasting > Targets

A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect.

And Spellcasting > Range

The target of a spell must be within the spell's range. For a spell like magic missile, the target is a creature. For a spell like fireball, the target is the point in space where the ball of fire erupts.

And per the rules of cover:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

For spells that have a target of something/someone, if you cannot see it (concealed), you cannot target it.

Area of effect spells have a point, or place you center the spell. This can be where a creature is standing, but to The Weave, it's just a point in space. So the point can't be somewhere concealed, but it will occur at the space you can see closest to that point. In this case, on the caster's side of the wall.

• You cannot Magic Missile a target that is hiding in a Darkness spell, but you can cast a Fireball in the middle and hope for the best.


• You cannot Fire Bolt the thief behind the barricade, but Sickening Radiance goes around corners.